Unexpected superhard phases of niobium triborides: first-principles calculations

Abstract

Using an unbiased structure search method based on particle-swarm optimization algorithms in combination with density functional theory calculations, we investigate the phase stability and electronic properties of NbB₃ under high pressures. By structure searching as implemented in the CALYPSO code, we obtained the most stable monoclinic phase (C2/c) and four metastable phases (P6₃/mmm, C2/m, Pmm2 and Im2) for NbB₃. Under high pressure, the C2/m structure transforms to Pmm2 phase and to Im2 phase at about 29 and 46 GPa, respectively. Surprisingly, the five phases of NbB₃ are all dynamically and mechanically stable at ambient conditions. The high bulk and shear modulus, and low Poisson's ratio for both phases in NbB₃, make it a promising low compressible material. Moreover, interestingly the hardness of the five phases of NbB₃ are very close to 40 GPa, in particular, the C2/m and Pmm2 phases have sufficient hardness (45 and 44 GPa, respectively), for it to be considered as a superhard material. All phases of NbB₃ with high hardness are stable due to the strong covalent bonding nature via electronic density of states and electron localization function analysis.